Nitrogen oxide emissions, principally nitrogen dioxide (NO2) and nitric oxide (NO) and referred to collectively as NOx, are a major environmental concern owing to their toxic and damaging nature. Of all nitrogen oxide emissions produced in the U.S., 55 percent are attributed to stationary sources, such as utility boilers, industrial boilers, gas turbines and stationary engines. The U.S. Environmental Protection Agency has promulgated New Source Performance Standards (NSPS) to define the limits of various pollutants permitted from new stationary sources.
Many times when fuels are burned, NOx levels exceeding NSPS occur. These violations of NSPS standards are becoming of increased concern. Three approaches that can be taken to reduce NOx emissions are: (1) Making changes before combustion; (2) Making modifications during combustion; and (3) Adding controls after combustion. Typical precombustion approaches are fuel switching, emulsifying the fuel with water, and denitrifying the fuel. Typical combustion modification techniques are changing stoichiometry, reducing temperature, and reducing residence time. Adding controls after combustion is generally referred to as flue-gas treatment.
NOx reduction during combustion has been employed since the early 1970's to obtain limited NOx emission reductions. It is the most common NOx emission reduction approach being used today to achieve moderate control. To obtain higher levels of NOx reduction, it is generally necessary to employ a flue-gas treatment approach, or a combination of approaches.
Flue-gas treatment processes are of two types, dry processes and wet processes. Some processes are designed for the simultaneous removal of NOx and SO2. Many of the flue-gas treatment processes have been developed in Japan, where NOx emission limits are generally stricter than in the United States.
Dry flue-gas treatment processes are normally preferred over wet processes because (1) they usually involve less equipment, and (2) they generally produce less waste that requires disposal. Most dry processes, however, share one characteristic with wet processes: they are both very expensive.
A number of dry processes are either commercially available or are well along in development. They range from catalytic and noncatalytic reduction to absorption processes and irradiation with electron beams. Today, the most popular flue-gas treatment process by far, at least for utility boilers, is selective catalytic reduction (SCR). In SCR, ammonia is used as a reducing agent. In the process, NOx is reduced to N2 and H2O by ammonia at 300 to 400 C. in the presence of a catalyst. Ammonia is an acceptable reducing agent for NOx in combustion gases because it selectively reacts with NOx while other reducing agents, such as H2, CO, and methane, readily react with O2 in the gases. The catalyst that are employed are normally precious metals, such as platinum, rhodium, palladium, ruthenium, osmium or iridium, or combinations thereof. SCR processes are very expensive primarily because of these precious metals that are required.
Recently, while conducting SCR experiments, the Inventor discovered that vermiculite, a low-cost silicate mineral, could be used as a catalyst instead of precious metals to achieve NOx reductions. Vermiculite has long been suspected of having catalytic properties, but a review of the literature has shown that no one has ever considered it as a NOx catalyst. Not only did the Inventor achieve good NOx reductions with ammonia injections and vermiculite, he also discovered that NOx reductions occurred over a wider temperature range than with typical SCR processes employing precious metal catalysts.